In piezoelectric motors, one or more piezoelectric elements are excited with electrical signals to extend and contract in order to generate a microscopic mechanical motion within the motor that is transformed into a macroscopic motion of a driven element. In part, piezoelectric motor designs differ in the electric signals used to excite the motions, in the form of the microscopic motion, and in the mechanism used to transform the microscopic motion into a macroscopic motion.
Piezoelectric motors take various forms and have various control systems. Some piezoelectric motors operate principally with sinusoidal electric signals of a single frequency, and are referred to here as single-frequency motors. The single frequency piezoelectric motors contrast with piezoelectric motors that require special waveforms for operation, such as triangular waveforms, such shaped waveforms having frequency spectra that are the composite of many frequencies with the overall result being a shaped waveform. Some single-frequency piezoelectric motors may also be operated with electrical signals that contain other frequency components, but it is not necessary to include additional frequency components for proper operation of a single-frequency piezoelectric motor. Single-frequency piezoelectric motors may also have more than one operational frequency that, when used at distinct times, result in distinct macroscopic motions of the driven element at those times. For example, U.S. Patent Publication No. 2002/0038987A1, the entire contents of which are hereby incorporated by reference, discloses embodiments that include single-frequency piezoelectric motors that have two distinct operational frequencies, one for a forward motion and one for a backward motion of a driven element.
The optimal frequency of operation, i.e., the frequency at which the mechanical output and performance of a piezoelectric motor is in some sense optimal, is typically related to a mechanical resonance. The optimal frequency therefore varies with several factors, such as temperature. Ambient temperature can change and vary the performance, and piezoelectric motors warm up during operation and that can affect performance. Further effects that influence the optimal frequency of a piezoelectric motor during its lifetime include fatigue, wear such as abrasion between the piezoelectric motor and the driven element, and other factors. Furthermore, differences during manufacturing and assembly and general tolerances result in a different optimal frequency for any two piezoelectric motors of the same design and manufacture. Finally, even if the optimal operating frequency was known beforehand, it is not guaranteed that the electronic circuit supplying the electric signal is able to generate the optimal frequency exactly, since the circuitry itself is subject to effects of temperature changes, aging, and manufacturing tolerances.
There is thus a need for an electrical driving circuit that drives a piezoelectric motor at or near its optimal frequency of operation by employing means of control. Prior art includes Phase Locked Loop (PLL) feedback control solutions. It is known that when a typical piezoelectric motor is excited close to its operational resonance frequency, there occurs a phase difference between the excitation signal and the vibration of the piezoelectric motor. If the vibration can be measured, a PLL may be able to exploit this phase difference and continuously track the operation frequency of the piezoelectric motor. PLL requires a dedicated continuously operating control circuit, and it is limited by the frequency range in which a phase difference is discernible, and is further limited by various electrical noise factors. PLL works only for piezoelectric motors where there is a clear monotonous relationship between the measured phase difference and the quality (strength, speed, etc.) of the resulting macroscopic motion. This relationship may not exist for all piezoelectric motor designs.
There is thus a need for control schemes that can drive a single-frequency piezoelectric motor sufficiently near its optimal frequency of operation but that are less dependent on the particularities of the piezoelectric motor and that can accommodate more variation in the piezoelectric motor design and manufacture.